LibWeb: Add a bare-bones HTML event loop with a task queue

This patch attaches a HTML::EventLoop to the main thread JS::VM used
for JavaScript bindings in the web engine.

The goal here is to model the various task scheduling mechanisms of the
HTML specification.

Userland/Libraries/LibWeb/HTML/EventLoop/EventLoop.h

@@ -0,0 +1,28 @@
+/*
+ * Copyright (c) 2021, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/Function.h>
+#include <LibWeb/HTML/EventLoop/TaskQueue.h>
+
+namespace Web::HTML {
+
+class EventLoop {
+public:
+    EventLoop();
+    ~EventLoop();
+
+    TaskQueue& task_queue() { return m_task_queue; }
+    TaskQueue const& task_queue() const { return m_task_queue; }
+
+private:
+    TaskQueue m_task_queue;
+};
+
+EventLoop& main_thread_event_loop();
+
+}